Transistor with variable channel width

ABSTRACT

A gate insulating film  103  is oxidized by a thermal oxidation method using a gate electrode  104  as a mask. At this time, the thickness of the gate insulating film  103  becomes thicker so that the portions indicated by  106  and  107  are obtained. The thickness of an active layer becomes thin at an end  112  of a channel, so that the distance from the gate electrode becomes long by the thickness. Then the strength of an electric field between a source and drain is relaxed by this portion. In this way, a thin film transistor having improved withstand voltage characteristics and leak current characteristics is obtained.

This application is a Divisional of application Ser. No. 09/110,894filed Jul. 7, 1998. U.S. Pat. No. 6,013,929.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a structure of a thin film transistorin which its withstand voltage characteristics and leak currentcharacteristics are improved. The present invention also relates to amethod of manufacturing the thin film transistor.

2. Description of the Related Art

In recent years, there has been known a technique in which a thin filmsemiconductor film is formed on a glass substrate or a quartz substrateand a transistor is manufactured by using the film as an active layer.This transistor is generically referred to as a thin film transistor.The thin film transistor will be hereinafter referred to as a TFT.

In general, as a semiconductor thin film for forming an active layer ofa TFT, an amorphous silicon film or a crystalline silicon film(polycrystalline film or microcrystalline film) is used. This is becausea single crystal silicon film can not be formed on a glass substrate ora quartz substrate by an existing technique.

Although low withstand voltage and large OFF-state current value do notbecome a subject of discussion in a TFT using an amorphous silicon filmsince its total characteristics are also low, they become a problem in aTFT using a crystalline silicon film.

This problem is caused because the density of defects existing in thesilicon film is extremely high as compared with single crystal silicon.

As means for solving the problem, there are known structures disclosedin Japanese Patent Publication No. Hei. 3-38755 and Japanese PatentUnexamined Publication Nos. Hei. 4-360580 and Hei. 5-166837.

The structures disclosed in the above publications are referred to as anLDD technique or an offset technique. In these techniques, a highresistance region, which does not function as a channel or a drain, isdisposed between a channel region and a drain region, so that a highelectric filed applied between the channel region and the drain regionis relaxed.

At an OFF operation, the movement of carriers via defects present in thevicinity of the boundary between the channel region and the drain regionis suppressed.

The type of high resistance region is roughly divided into a structureof a non-doped region (generically referred to as an offset structure)and a structure of a lightly doped region (generically referred to as anLDD structure).

Japanese Patent Unexamined Publication Nos. Hei. 4-360580 and Hei.5-166837 disclose a technique as a method of forming a high resistanceregion in which an anodic oxidation film is formed on the surface of agate electrode and the high resistance region is formed in aself-aligning manner by the thickness of the anodic oxidation film.

This method has a feature that the high resistance region can be formedwith high controllability.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a TFT with a novelstructure in which its withstand voltage is high and OFF-state currentis small.

According to a first aspect of the present invention, a thin filmtransistor comprises an active layer composed of some regions, a gateinsulating film of a silicon oxide film formed on the active layer, anda gate electrode formed on the gate insulting film, and is characterizedin that the active layer has been subjected to oxidation using the gateelectrode as a mask, and the shape of a certain portion of the gateinsulating film is different from that of the other portion thereof.

The certain portion is formed by thermal oxidation using the gateelectrode as a mask. In this portion, the thermal oxidation in theactive layer progresses to a region under the gate electrode.

In this state, the end of the upper portion of a channel region formedin the active layer is selectively oxidized. By making such a structure,it is possible to realize the state in which the thickness of the gateinsulating film at the end of the upper portion of the channel graduallybecomes thicker in the direction from the channel to a source or fromthe channel to a drain.

According to another aspect of the present invention, a method ofmanufacturing a thin film transistor comprises the steps of: forming asilicon oxide film on an active layer; forming a gate electrode on thesilicon oxide film; and carrying out thermal oxidation using the gateelectrode as a mask to increase the thickness of the silicon oxide filmexcept a portion; and is characterized in that the end of the regionsubjected to the thermal oxidation extends to the portion under the gateelectrode.

In this structure, the end of the upper portion of the channel region isselectively oxidized. That is, the thermal oxidation progresses to theend of the upper portion of the channel region.

By adopting this structure, the thickness of the gate insulating film atthe end of the upper portion of the channel region is gradually changed.That is, it is possible to obtain such a structure that the thickness ofthe gate insulating film gradually becomes thicker in the direction fromthe end of the upper portion of the channel region to the source/drainregions.

The silicon oxide film prior to the thermal oxidation may be a siliconoxide film having an incomplete constitution. This film may contain anelement other than oxygen and silicon if the film has such a filmquality that thermal oxidation progresses.

In the manufacturing steps of the present invention, after the gateinsulating film of the silicon oxide film is formed, the gate electrodeis formed, and then the thermal oxidation is carried out.

By this, the thermal oxidation progresses in the portion except theregion where the gate electrode is disposed, and the thickness of theportion becomes thick. The thermal oxidation also progresses to aportion under the gate electrode.

In this way, a state in which the thickness of the gate insulating filmis gradually changed, is obtained in this portion. That is, there isobtained such a state that the thickness of the gate insulating filmbecomes thicker in the direction from the end of the upper portion ofthe channel region to the source/drain regions.

By this, it is possible to realize the structure in which the electricfield applied to the active layer from the gate electrode is graduallyweakened correspondingly to the change of the thickness of the gateinsulating film in the direction from the end of the upper portion ofthe channel region to the source/drain regions.

The inversion layer formed on the end of the upper portion of thechannel region can be made weak as compared with that on the center ofthe channel, and there is obtained an effect similar to the case where alightly doped region or an offset region is formed.

That is, it is possible to realize the structure in which the electricfield applied between the channel region and the drain region can bemade weak.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are views showing manufacturing steps of a TFT ofEmbodiment 1.

FIGS. 2A to 2E are views showing manufacturing steps of a TFT ofEmbodiment 3.

FIGS. 3A to 3F are schematic views showing examples of devices usingTFTs of Embodiment 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[Embodiment 1]

FIGS. 1A to 1D show manufacturing steps in this embodiment. First, anot-shown amorphous silicon film with a thickness of 50 nm is formed ona quartz substrate 101 by a low pressure CVD method. It is suitable toselect the thickness of the silicon film within the range of 20 nm to100 nm. Next, the amorphous silicon film is crystallized by a heattreatment to obtain a crystalline silicon film.

The thus obtained crystalline silicon film is patterned to obtain anactive layer pattern 102 shown in FIG. 1A.

Next, a silicon oxide film 103 with a thickness of 30 nm as the originof a gate insulating film is formed by a plasma CVD method. Here, thefilm growth is carried out by using SiH₄, N₂ O and O₂ as raw materialgas.

Next, a tantalum film with a thickness of 400 nm is formed by asputtering method. The tantalum film is patterned to form a pattern 104.Further, an anodic oxidation film 105 is formed on the circumferentialsurface of the tantalum pattern 104 (FIG. 1A).

Here, the thickness of the anodic oxidation film is made 80 nm. In thisway, the gate electrode 104 having the anodic oxidation film 105 formedon the circumferential surface thereof is obtained (FIG. 1A).

Next, in an oxygen atmosphere, a heat treatment at 950°C. for 30 minutesis carried out. In this step, thermal oxidation progresses mainly in anexposed portion of the silicon oxide film 103 so that a gate insulatingfilm 100 including thick oxide films 106 and 107 is obtained.

The thermal oxidation progresses so as to intrude into the periphery ofa portion under the gate electrode 104 as indicated by 108. That is, thethermal oxidation also progresses into the portion of the active layerat the side end lower portion of the gate electrode (FIG. 1B).

The feature in this state is that the thickness of the gate insulatingfilm is gradually increased in the direction from the side end lowerportion 108 of the gate electrode to the upper portion of a sourceregion and the upper portion of a drain region.

After the state shown in FIG. 1B is obtained, doping of phosphorus iscarried out by using the gate electrode 104 as a mask. Here, the dopingof phosphorus is carried out by a plasma doping method. In thisembodiment, since an N-channel TFT is fabricated, the doping ofphosphorus is carried out. If a P-channel TFT is fabricated, doping ofboron is carried out (FIG. 1C).

In this step, a source region 109, a channel region 110, and a drainregion 111 are formed in a self-aligning manner.

Although the drawing shows as if the position of the interface betweenthe source region 109 and the channel region 110, and the interfacebetween the drain region 111 and the channel region 110 substantiallycoincides with the position of the surface of the anodic oxidation film105 formed on the side of the gate electrode 104, the position of theinterface becomes slightly different by the intrusion of ions orconditions of a subsequent activation step.

After the doping of phosphorus is ended, laser beam irradiation iscarried out to anneal damages of a crystal structure generated at thedoping and to activate dopants.

In the structure shown in this embodiment, in a portion 112, that is, inthe interface between the source region 109 and the channel region 110and its vicinities, and in the interface between the drain region 111and the channel region 110 and its vicinities, such a structure isobtained that the thickness of a gate insulating film 100 graduallybecomes thicker toward the source/drain regions.

By making such a structure, the electric field from the gate electrodeis gradually weakened toward the source/drain regions in the interfacesbetween the channel region and the source/drain regions and thevicinities thereof.

In this structure, in an OFF state for example, the resistance of aportion of the channel region adjacent to the source/drain regions(portion at the end of the channel) becomes high as compared with thecenter of the channel. This portion functions as a high resistanceregion like an offset region or an LDD region.

That is, by making the resistance of the portion of the channel regionadjacent to the source/drain regions high as compared with the center ofthe channel, it is possible to relax a high electric field appliedbetween the channel region and the drain region at an OFF operation.

In this way, it is possible to realize the structure in which anOFF-state current is lowered.

The above-mentioned function also operates at an ON operation, and inthis case, there is obtained an effect to increase the withstandvoltage.

After the state shown in FIG. 1C is obtained, as shown in FIG. 1D, asilicon nitride film 113 with a thickness of 150 nm is formed as aninterlayer insulating film by a plasma CVD method, and further, anacrylic resin film 114 is formed.

The thickness of the acrylic resin film is adjusted so that the minimumthickness becomes 700 nm. Here, the acrylic resin film is used to securethe flatness of the surface. Polyimide, polyamide, polyimide amide,epoxy, etc. may be used other than acryl.

Next, contact holes are formed, and a source electrode 115 and a drainelectrode 116 are formed. In this way, the N-channel TFF is completed.

[Embodiment 2]

In this embodiment, in the structure shown in embodiment 1, thethickness of the anodic oxidation film 105 is made as thick as 200 nm.In this case, it becomes impossible to neglect the role that the filmthickness functions as a mask at doping. By the film thickness, offsetgate regions are formed between the channel region and the source/drainregions.

That is, the offset regions are formed adjacently to the channel region,and the source/drain regions are formed adjacently to the offsetregions.

In the structure shown in this embodiment, just in the portion where theoffset region is formed, the thickness of the gate insulating filmgradually becomes thick from the end of the upper portion of the channelregion to the source/drain regions.

In this structure, in addition to the effect obtained from the existenceof the offset region, the effect of the structure shown in embodiment 1can be obtained. The withstand voltage and OFF current characteristicscan be further improved.

[Embodiment 3]

In this embodiment, in the manufacturing steps shown in embodiment 1,after the silicon oxide film 103 is formed on the active layer 102 andthe gate electrode 104 is formed, a step in which the silicon oxide film103 is patterned by dry etching is introduced.

FIGS. 2A to 2E show the manufacturing steps in this embodiment. First,similarly to embodiment 1, a not-shown amorphous silicon film with athickness of 50 nm is formed on a quartz substrate 201 by a low pressureCVD method. It is suitable to select the thickness of the silicon filmwithin the range of 20 nm to 100 nm. Next, the amorphous silicon film iscrystallized by a heat treatment to obtain a crystalline silicon film.

The thus obtained crystalline silicon film is patterned to obtain anactive layer pattern 202 shown in FIG. 2A.

Next, a silicon oxide film 203 with a thickness of 30 nm as the originof a gate insulating film is formed by a plasma CVD method. Here, SiH₄,N₂O and O₂ are used as raw material gas.

Next, a tantalum film with a thickness of 400 nm is formed by asputtering method. The tantalum film is patterned to form a pattern 204.Further, an anodic oxidation film 205 is formed on the circumferentialsurface of the tantalum pattern 204 by an anodic oxidation method.

Here, similarly to embodiment 1, the thickness of the anodic oxidationfilm 205 is made 80 nm. In this way, the gate electrode 204 having theanodic oxidation film 205 formed on the circumferential surface thereofis obtained (FIG. 2A).

Next, by using the gate electrode 204 as a mask, the silicon oxide film203 is dry etched by CHF₃ and is patterned, so that a silicon oxide film206 is obtained (FIG. 2B).

Next, in an oxygen atmosphere, a heat treatment at 950° C. for 30minutes is carried out. In this step, thermal oxidation newly progressesin the exposed portion of the active layer pattern 202 of thecrystalline silicon film so that a gate insulating film 200 includingsuch regions indicated by 207 and 208 is obtained.

The thermal oxidation progresses so as to intrude into the periphery ofa portion under the side end of the gate electrode 204, as indicated by209. That is, the thermal oxidation also progresses into the portion ofthe active layer positioned at the side end lower portion of the siliconoxide film 206, so that the silicon oxide film 206 and the gateinsulating film 200 are integrated with each other and the state shownin FIG. 2C is obtained.

The feature in this state is that the thickness of the gate insulatingfilm gradually becomes thicker from the side end lower portion 209 ofthe gate electrode to the upper portion of a source region and the upperportion of a drain region.

After the state shown in FIG. 2C is obtained, doping of phosphorus iscarried out by using the gate electrode 204 as a mask. Here, similarlyto embodiment 1, the doping of phosphorus is carried out by a plasmadoping method. In this embodiment, since an N-type TFT is fabricated,the doping of phosphorus is carried out. If a P-channel TFT isfabricated, doping of boron is carried out (FIG. 2D).

In this step, a source region 210, a channel region 211, and a drainregion 212 are formed in a self-aligning manner.

In the drawing, it is shown as if the position of the interface betweenthe source region 210 and the channel region 211, and the interfacebetween the drain region 212 and the channel region 211 substantiallycoincides with the position of the surface of the anodic oxidation film205 formed on the side of the gate electrode 204. However, the positionof the interface becomes slightly different by the intrusion of ions orconditions of a subsequent activation step.

After the doping of phosphorus is ended, laser beam irradiation iscarried out so that damages of a crystal structure generated at thedoping are annealed and the dopants are activated.

In the structure shown in this embodiment, the thickness of the gateinsulating film 200 gradually becomes thicker in portions 213, that is,in the interface between the source region 210 and the channel region211 and its vicinities, and in the interface between the drain region212 and the channel region 211 and its vicinities.

By such a structure, it is possible to gradually weaken an electricfield, which is applied from the gate electrode, toward the direction ofsource/drain in the interfaces between the channel region and thesource/drain regions and the vicinities thereof.

According to this structure, in an OFF state for example, the resistanceof a portion of the channel region adjacent to the source/drain regions(portion at the end of the channel region) becomes high as compared withthe center of the channel. This portion functions as a high resistanceregion like an offset region or an LDD region.

That is, the resistance of the portion of the channel region adjacent tothe source/drain regions is made high as compared with the center of thechannel, so that it is possible to relax a high electric field appliedbetween the channel region and the drain region at an OFF operation.

In this way, it is possible to make the structure in which an OFF-statecurrent is lowered.

The above-mentioned function also operates at an ON operation, and inthis case, the effect of increasing the withstand voltage can beobtained.

After the state shown in FIG. 2D is obtained, a silicon nitride film 214with a thickness of 150 nm is formed as an interlayer insulating film bya plasma CVD method. Further, an acrylic resin film 215 is formed.

The thickness of the acrylic resin film is adjusted so that the minimumthickness becomes 700 nm. Here, the acrylic resin film is used to securethe flatness of the surface. Polyimide, polyamide, polyimide amide,epoxy, etc. may be used other than acryl.

Next, contact holes are formed, and a source electrode 216 and a drainelectrode 217 are formed. In this way, the N-channel TFT is completed.

In this embodiment, since the gate insulating film 200 is thinner thanthe case of embodiment 1, it is possible to make the depth of the formedcontact hole shorter than embodiment 1.

[Embodiment 4]

In this embodiment, in the structure shown in embodiment 1 or 3, asilicon material given a conductivity type is used as a material of agate electrode.

Various kinds of silicide materials or metallic materials may be usedother than silicon materials. For example, tungsten silicide, molybdenumsilicide, and the like may be used. However, since a step of forming athermal oxidation film is required after the formation of the gateelectrode, it is necessary to use a material for the gate electrode,which can withstand heating at thermal oxidation.

[Embodiment 5]

The thin film transistor disclosed in the present specification can beused for various kinds of thin film integrated circuits, various kindsof flat panel displays, an information processing terminal equipped witha flat panel display, a video camera, and the like. In the presentspecification, these devices will be generically referred to as asemiconductor device.

Examples of the specific structures of various semiconductor deviceswill be described below. FIGS. 3A to 3F show examples of the variouskinds of semiconductor devices. Each of these semiconductor devices usesthe TFT for at least a part thereof.

FIG. 3A shows a portable information processing terminal. Thisinformation processing terminal includes a main body 2001 equipped withan active matrix type liquid crystal display or an active matrix type ELdisplay 2005 and a camera portion 2002 for taking in information fromthe outside.

An image receiving portion 2003 and an operation switch 2004 aredisposed at the camera portion 2002.

It is supposed that in future, the information processing terminalbecomes increasingly thin and light to improve the portability.

In such a structure, it is preferable to integrate a peripheral drivingcircuit, an arithmetic circuit, and a memory circuit by TFTs on asubstrate where the active matrix type display 2005 is formed.

FIG. 3B shows a head mount display. This device includes a main body2101 equipped with an active matrix type liquid crystal display or an ELdisplay 2102. The main body 2101 can be mounted on a head by a band2103.

FIG. 3C shows a projection type liquid crystal display device which is adevice referred to as a front projection type.

This device has such a function that light from a light source 2202installed in a main body 2201 is optically modulated by a refection typeliquid crystal display device 2203, and is enlarged by an optical system2204 so that an image is projected onto a screen 2205.

In such a structure, the optical system 2204 is required to beminiaturized to the utmost in view of cost. Correspondingly to therequirement, the display device 2203 is also required to beminiaturized.

In the case where an active matrix type flat panel display isminiaturized, it is required that a peripheral driving circuit fordriving the active matrix circuit is also integrated on the samesubstrate as the active matrix circuit.

This is because when the active matrix circuit is miniaturized, even ifa circuit for constituting the peripheral driving circuit is formed byan externally mounted IC, it becomes difficult to install the IC.

Thus, in the display device 2203, such a structure is adopted that theactive matrix circuit and peripheral driving circuit are integrated onthe same substrate by TFTs.

In this example, a reflection type is used for the liquid crystaldisplay device 2203. However, a transmission type liquid crystal displaydevice may be used. In this case, an optical system becomes different.

FIG. 3D shows a portable telephone. This device includes a main body2301 equipped with an active matrix type liquid crystal display device2304, an operation switch 2305, an audio input portion 2303, an audiooutput portion 2302, and an antenna 2306.

In recent years, a structure such as a combination of the portableinformation processing terminal as shown in FIG. 3A and the portabletelephone as shown in FIG. 3D has been produced on a commercial basis.

FIG. 3E shows a portable video camera. This device includes a main body2401 equipped with an image receiving portion 2406, an audio inputportion 2403, an operation switch 2404, an active matrix type liquidcrystal display 2402, and a battery 2405.

FIG. 3F shows a rear projection type liquid crystal display device. Thisstructure is such that a main body 2501 is equipped with a screen forprojection. Displaying is made in such a way that light from a lightsource 2502 is separated by a polarizing beam splitter 2504, theseparated light is optically modulated by a reflection type liquidcrystal display device 2503, the optically modulated light is reflectedby reflectors 2505 and 2506, and the reflected light is projected onto ascreen 2507.

In this example, a reflection type is used for the liquid crystaldisplay device 2503. However, a transmission type liquid crystal displaydevice may be used. In this case, it is sufficient if the optical systemis changed.

In these examples, liquid crystal display devices have been mainlyshown. However, an EL display device may be adopted as an active matrixtype display device.

According to the present invention disclosed in the presentspecification, it is possible to provide a TFT having high withstandvoltage and small OFF-state current.

What is claimed is:
 1. A semiconductor device comprising: an activelayer formed over a substrate, said active layer having source and drainregions, and a channel region therebetween; a gate insulating filmformed on said active layer; and a gate electrode formed on said gateinsulating film, wherein a thickness of said active layer graduallybecomes thinner in a direction from said channel region toward saidsource and drain regions, wherein the thickness of said active layer atside end portions of said channel region is thinner than that at aninner portion of said channel region.
 2. A semiconductor deviceaccording to claim 1, wherein said gate electrode comprises a materialwhich is able to be anodized and to withstand a thermal oxidation step.3. A semiconductor device according to claim 1, wherein an upper endportion of said channel region is selectively oxidized.
 4. Asemiconductor device according to claim 1, wherein said semiconductordevice is selected from the group consisting of a portable informationterminal, a portable phone, and a portable video camera.
 5. Asemiconductor device comprising: an active layer formed over asubstrate, said active layer having source and drain regions, and achannel region therebetween; a gate insulating film comprising siliconoxide formed on said active layer; and a gate electrode formed on saidgate insulating film, wherein a thickness of said active layer graduallybecomes thinner in a direction from said channel region toward saidsource and drain regions, wherein the thickness of said active layer atside end portions of said channel region is thinner than that at aninner portion of said channel region.
 6. A semiconductor deviceaccording to claim 5, wherein said gate electrode comprises a materialwhich is able to be anodized and to withstand a thermal oxidation step.7. A semiconductor device according to claim 5, wherein an upper endportion of said channel region is selectively oxidized.
 8. Asemiconductor device according to claim 5, wherein said semiconductordevice is selected from the group consisting of a portable informationterminal, a portable phone, and a portable video camera.
 9. Asemiconductor device comprising: an active layer formed over asubstrate, said active layer having source and drain regions, and achannel region therebetween; a gate insulating film formed on saidactive layer; a gate electrode formed on said gate insulating film; ainsulating film comprising silicon nitride formed over said gateinsulating film; a leveling film comprising organic resin formed on saidinsulating film; a source electrode formed on said leveling film, saidsource electrode electrically connected to said source region; and adrain electrode formed on said leveling film, said drain electrodeelectrically connected to said drain region, wherein a thickness of saidactive layer gradually becomes thinner in a direction from said channelregion toward said source and drain regions, wherein the thickness ofsaid active layer at side end portions of said channel region is thinnerthan that at an inner portion of said channel region.
 10. Asemiconductor device according to claim 9, wherein said gate electrodecomprises a material which is able to be anodized and to withstand athermal oxidation step.
 11. A semiconductor device according to claim 9,wherein an upper end portion of said channel region is selectivelyoxidized.
 12. A semiconductor device according to claim 9, wherein saidsemiconductor device is selected from the group consisting of a portableinformation terminal, a portable phone, and a portable video camera. 13.A semiconductor device according to claim 9, wherein said organic resinis selected from the group consisting of acryl, polyimide, polyamide,polyimideamide and epoxy.
 14. A projector comprising: a display devicehaving at least one thin film transistor, said thin film transistorincluding: an active layer formed over a substrate, said active layerhaving source and drain regions, and a channel region therebetween; agate insulating film formed on said active layer; and a gate electrodeformed on said gate insulating film, wherein a thickness of said activelayer gradually becomes thinner in a direction from said channel regiontoward said source and drain regions, wherein the thickness of saidactive layer at side end portions of said channel region is thinner thanthat at an inner portion of said channel region.
 15. A projectoraccording to claim 14, wherein said gate electrode comprises a materialwhich is able to be anodized and to withstand a thermal oxidation step.16. A projector according to claim 14, wherein an upper end portion ofsaid channel region is selectively oxidized.
 17. A projector accordingto claim 14, wherein said display device is selected from the groupconsisting of a liquid crystal display device and an electroluminescencedisplay device.
 18. A projector comprising: a display device having atleast one thin film transistor, said thin film transistor including: anactive layer formed over a substrate, said active layer having sourceand drain regions, and a channel region therebetween; a gate insulatingfilm comprising silicon oxide formed on said active layer; and a gateelectrode formed on said gate insulating film, wherein a thickness ofsaid active layer gradually becomes thinner in a direction from saidchannel region toward said source and drain regions, wherein thethickness of said active layer at side end portions of said channelregion is thinner than that at an inner portion of said channel region.19. A projector according to claim 18, wherein said gate electrodecomprises a material which is able to be anodized and to withstand athermal oxidation step.
 20. A projector according to claim 18, whereinan upper end portion of said channel region is selectively oxidized. 21.A projector according to claim 18, wherein said display device isselected from the group consisting of a liquid crystal display deviceand an electroluminescence display device.
 22. A projector comprising: adisplay device having at least one thin film transistor, said thin filmtransistor including: an active layer formed over a substrate, saidactive layer having source and drain regions, and a channel regiontherebetween; a gate insulating film formed over said active layer; agate electrode formed on said gate insulating film; a insulating filmcomprising silicon nitride formed over said gate insulating film; aleveling film comprising organic resin formed on said insulating film; asource electrode formed on said leveling film, said source electrodeelectrically connected to said source region; and a drain electrodeformed on said leveling film, said drain electrode electricallyconnected to said drain region, wherein a thickness of said active layergradually becomes thinner in a direction from said channel region towardsaid source and drain regions, wherein the thickness of said activelayer at side end portions of said channel region is thinner than thatat an inner portion of said channel region.
 23. A projector according toclaim 22, wherein said gate electrode comprises a material which is ableto be anodized and to withstand a thermal oxidation step.
 24. Aprojector according to claim 22, wherein an upper end portion of saidchannel region is selectively oxidized.
 25. A projector according toclaim 22, wherein said display device is selected from the groupconsisting of a liquid crystal display device and an electroluminescencedisplay device.
 26. A projector according to claim 22, wherein saidorganic resin is selected from the group consisting of acryl, polyimide,polyamide, polyimideamide and epoxy.